The growth of mobile radio applications has led to the development of services using a variety of different air interface standards and radio frequency bands in different parts of the world. A current generation mobile phone is likely to provide for transmissions using the GSM or UMTS air interfaces (as defined by the international standards body 3GPP) on the 850 MHz, 900 MHz, 1800 MHz, 1900 MHz and 2100 MHz frequency bands. The development of compact antennas capable of operating on all these bands, for use in mobile handsets, laptop computers, trackers and other user equipment (UE) is very challenging. The development of antenna techniques has in general been evolutionary, simple dual band structures being progressively optimized to provide wider operating bandwidths at each of the two frequency bands. Current ‘pentaband’ antennas operate over the frequency bands 826-960 MHz and 1710-2170 MHz.
The economics of handset design and production, as well as users' requirements for world-wide roaming, imply that a handset is required to operate on all the standard frequency bands associated with the interface protocol(s) which it supports.
The advent of new mobile services in the frequency band 698-798 MHz, when combined with existing requirements in the band 826-960 MHz creates a new challenge to the antenna designer. The present invention provides a means by which this requirement may be satisfied without any significant increase in the volume occupied by the antenna.
With reference to FIG. 1, it is well known that a single radiating element 10 may be fed concurrently with radio signals at two frequencies, f1 and f2 by the means shown in FIG. 1, where 11 is a band-stop filter tuned to f2, 12 is a band-stop filter tuned to f1, 13 is an input matching circuit adjusted to provide the required matched input impedance at f1 and 14 is an input matching circuit adjusted to provide the required matched input impedance at f2. Such an arrangement works well if the bandwidths of the signals at f1 and f2 are small compared with their frequency separation (f1-f2). If the frequency separation is small or the bandwidth is large, then the design of suitable filters and matching circuits becomes difficult—their cost, dimensions and associated transmission losses become unacceptably large.
Alternative arrangements providing for optional transmission at f1 or f2 may be designed as shown in FIG. 2 by making use of a switch 15 at the antenna input and two alternative matching circuits, one for f1 [13] and the other for f2 [14]. Such an arrangement is satisfactory in many circumstances, but presupposes that the antenna may be matched effectively and economically for both frequency bands f1 and f2 when the feed point to the antenna is at one fixed location.
In the case of mobile radio antennas, the large width of the frequency bands in which f1 and f2 may be positioned, the small fractional separation between the adjacent ends of these frequency bands, and the necessarily small physical dimensions of the antenna (typically 0.2×0.06×0.025 wavelengths) result in an input impedance which is very difficult to match effectively over the specified bands. The result of inadequate impedance matching is reduced antenna efficiency with consequential reduced range, data rate and battery life.